1. Field of the Invention
The present invention relates to a method for producing water (heavy oxygen water) highly enriched in .sup.17 O or .sup.18 O (hereinafter referred to as the heavy isotopes of oxygen) which are isotopes of oxygen present in water. More specifically, the present invention relates to a method in which oxygen is enriched in .sup.16 O.sup.17 O, .sup.16 O.sup.18 O, .sup.17 O.sup.17 O, .sup.17 O.sup.18 O, and .sup.18 O.sup.18 O by means of cryogenic distillation, then water is produced from the enriched product, and then water which is further enriched in the oxygen isotope .sup.17 O or .sup.18 O is produced by means of water distillation.
In addition, the present invention relates to a method of producing water which is enriched in the oxygen isotope .sup.17 O or .sup.18 O, and which has a constitution in which the hydrogen isotopes are present in a ratio approximating their natural abundance ratio. In addition, alternatively, the present invention relates to a method of producing water having a hydrogen isotope constitution which contains deuterium (D) in a ratio lower than the natural abundance ratio. In addition, alternatively, the present invention relates to a method of producing water having a hydrogen isotope constitution which contains deuterium (D) in a ratio higher than the natural abundance ratio.
This application is based on patent application No. Hei 11-23148 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Natural oxygen contains .sup.16 O at a ratio of 99.7591% (atomic percent, hereinafter the same), .sup.17 O at a ratio of 0.037%, and .sup.18 O at a ratio of 0.204%.
Among these, the heavy isotope .sup.18 O is used as a tracer in fields such as agriculture, biology, and medicine.
In addition, in the same way, since the heavy isotope .sup.17 O has nuclear magnetic moment, it is used in research of oxygen compounds using nuclear magnetic resonance and the like.
As enrichment methods for these heavy isotopes of oxygen, there are distillation, thermal diffusion, chemical exchange (reactions), and the like. However, as a method of production with low cost and high volume, distillation is generally used. As the distillation method, there are methods which use water, NO, CO or oxygen as the starting material.
However, the above mentioned conventional techniques have the following problems.
In the above-mentioned water distillation method, it is easy for water (HD.sup.16 O) containing the light isotope (.sup.16 O) and deuterium to be mixed with the heavy isotope-enriched product.
FIG. 14 shows an example of enrichment in heavy isotopes of oxygen by means of water distillation using water containing oxygen in which each of the isotopes is present in the above-mentioned natural abundance ratios. In this example, the fraction obtained at the bottom of the column which was enriched in H.sub.2.sup.18 O to approximately 1.1% was also enriched in HD.sup.16 O to approximately 0.2%.
Since the relative volatility of H.sub.2.sup.16 O/HD.sup.16 O is greater than the relative volatility of H.sub.2.sup.16 O/H.sub.2.sup.18 O, enrichment in HD.sup.16 O is comparatively easier to carry out. Therefore, it is believed that in the above-mentioned example, enrichment in HD.sup.16 O hinders enrichment in H.sub.2.sup.17 O and H.sub.2.sup.18 O.
Dostrovsky et al reported obtaining H.sub.2.sup.18 O of 99% or greater by means of water distillation (I. Dostrovsky and M. Epstein, "The production of stable isotopes of oxygen" Analytical Chemistry Symposia Series, Vol. 11, pp. 693-702 (1982)). However, as mentioned above, when enriching in H.sub.2.sup.17 O and H.sub.2.sup.18 O, usually, enrichment in water molecules (HD.sup.16 O, etc.) containing .sup.16 O and deuterium also occurs at the same time, this hinders the enrichment in H.sub.2.sup.17 O and H.sub.2.sup.18 O, and it is difficult to industrially obtain H.sub.2.sup.18 O having a purity of 99% or greater. The purity of commercially available H.sub.2.sup.18 O is approximately 97%.
In addition, because the latent beat of vaporization of water is comparatively large (for example, it is about six times greater than that of oxygen), the apparatus for water distillation is comparatively large and energy consumption is great. For this reason, there is a tendency for the apparatus costs and operation costs to be large for water distillation methods.
In NO distillation methods as well, there is the problem that it is relatively easy for the compound NO (.sup.15 N.sup.16 O) which contains the heavy isotope of nitrogen and the .sup.16 O to be mixed with the obtained heavy isotope-enriched product, and it is difficult to obtain an enriched product having a high concentration of heavy oxygen isotopes.
In oxygen distillation, since the abundance ratio of .sup.17 O and .sup.18 O in natural oxygen is low, the abundance ratio of .sup.17 O.sup.17 O, .sup.17 O.sup.18 O and .sup.18 O.sup.18 O is extremely low. For this reason, the major proportion of oxygen molecules which contain heavy isotopes are .sup.16 O.sup.17 O and .sup.16 O.sup.18 O.
In this way, since most heavy isotope containing-oxygen molecules contain .sup.16 O, in oxygen distillation, even when the enrichment proceeds to close to 100% of heavy isotope containing-oxygen molecules, the enrichment rate for the heavy isotopes, that is .sup.17 O and .sup.18 O, is a low value of 50% or less.